A fuel cell is a device that converts the chemical energy from a fuel such as hydrogen or methane into electricity. Recently, it draws attentions as a possible solution to ecofriendly vehicles. Also, with increased possibility of application as power source for houses, mobile devices, etc., researches are actively carried out.
Researches on fuel cells are focused on improvement of output density and output voltage by improving the characteristics of electrode, fuel and electrolyte membrane. In particular, many attempts are made to improve the activity of catalyst for improvement of electrode.
The electrode performance of a fuel cell is largely dependent on the chemical composition, size, distribution, stability, etc. of electrode catalyst nanoparticles that catalyzes oxidation and reduction reactions on the electrode. Also, it is greatly affected by the easiness of material transfer, such as diffusion of reactants to a catalyst layer, discharge of products, etc., which is related with the reactive surface area as well as the structure, distribution and connectivity of pores of the catalyst layer.
The slow rate of oxygen reduction reaction (ORR) is the major obstacle of the application of a polymer exchange membrane fuel cell (PEMFC) to vehicles, power generators, etc.
For utilization of the PEMFC in such applications as vehicles and power generators, the kinetic limitation of ORR has to be overcome since it is the fundamental cause of the following three problems.
Firstly, the overpotential necessary for the ORR reduces thermal efficiency under practical operation current densities much lower than its thermodynamic limit. Secondly, in order to satisfy the cost requirement for application to vehicles and power generators, the loading amount of platinum (Pt) on a PEMFC stack has to be decreased to about ⅕. Lastly, the loss and/or decomposition of Pt in a cathode has to be substantially decreased.
If a stable cathode catalyst having a specific activity somewhat increased over the up-to-date Pt/C catalyst could be developed, these limitations might be overcome. With the hope that improved catalytic activity and stability could be achieved from combinations of different metals, many binary alloys (or multicomponent) metal alloys are studied for ORR. Although these efforts resulted in gradual improvement of catalytic performance, much more improvement is still needed.